Device Types

Various types of device are available for carrying out the switching and protection tasks listed under title that are specially designed to fulfill the respective requirements. The various parts of IEC 60947 (Low voltage switchgear and controlgear) specify the design, performance and test features of the devices.

Figure 1 – Switch symbols: The horizontal line in the switch symbol of the contacts indicates that they fulfill the isolating function

1. Disconnectors (isolating switches)

Legrand’s Isolating switches (photo credit: reliancegroupco.com)

The disconnector is a mechanical device that fulfills in the open position the requirements specified for the isolation function (IEC 60947-1).

The purpose of the isolating function is to cut off the supply from all or a discrete section of the installation by separating the installation or section from every source of electrical energy for reasons of safety.

The key factor here is the opening distance. Isolation must be guaranteed from pole to pole and from input to output, whether this is by means of a visible isolation gap or by suitable design features within the device (mechanical interlocking mechanism).

A device fulfills the isolating function stipulated under IEC 60947-1 when in the “Open” position the isolation at a defined withstand voltage is assured between the open contacts of the main circuit of the switchgear.

It must also be equipped with an indicator device in relation to the position of the movable contacts. This position indicator must be linked in a secure, reliable way to the actuator, whereby the position indicator can also serve as actuator, provided that it can only display the position “Open” in the “OFF” position, when all moving contacts are in the “Open” position. This is to be verified by testing.

According to IEC 60947-3, an isolator must only be able to make and break a circuit, if either a current of negligible size is switched on or off, or if during switching no noticeable voltage difference between the terminals of each pole occurs.

Under normal conditions it can conduct operational currents as well as under abnormal conditions larger currents (e.g. short-circuit currents) for a certain period.

The isolator function can be realized with a variety of devices such as for example in disconnectors, fuse-disconnectors, switch-disconnectors, fuse-switch disconnectors and circuit breakers with isolating function.

2. Load switches

Load switches (or only “switches”) are mechanical switching devices capable of making, carrying and breaking currents under normal circuit conditions which may include specified operating overload conditions and also carrying for a specified time currents under specified abnormal circuit conditions such as those of short-circuit.

A load switch may have a short-circuit making capacity, however it does not have a short-circuit breaking capacity (IEC 60947-1 and -3).

Short-circuit currents can be conducted (high short-circuit withstand capacity), but not be switched-off.

For load switches the range of designs is similarly wide as for isolator switches, for example “normal” (load) switches, fuse-switches, circuit breakers.

3. Switch disconnectors

Switch disconnectors combine the properties of (load) switches and disconnectors. In this case, too, there are a variety of designs such as “normal” switch disconnectors, fuse- switch-disconnectors and circuit breakers.

4. Circuit breakers

Circuit breakers are mechanical switching devices, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short-circuit (IEC 60947-2).

They thus also fulfill the requirements of (load) switches. Circuit breakers are often designed so that they can fulfill the requirements for disconnectors.

SEARCH: Articles, software & guides

Premium Membership

About Author

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on Google+

Premium Membership

Get access to specialized technical articles and premium electrical guides. 20% Off with promo code BF47F80C84.